Syngas (which is a short for “synthesis gas”) comprises carbon monoxide (CO) and hydrogen (H2), and is an important source for the production of H2. The conventional industrial process for producing syngas on an industrial scale is steam reforming, whereby a carbonaceous material is reacted with water in the form of steam. Another process, of increasing importance, involves subjecting the carbonaceous material to Catalytic Partial Oxidation. The present invention pertains to a process of this type, referred to hereinafter as “CPO process.”
A typical CPO process, used for producing H2, a carbonaceous material is subjected to catalytic partial oxidation, after which steam is added and a water gas shift reaction is allowed to take place. In this water gas shift reaction, carbon monoxide reacts with water so as to form carbon dioxide and hydrogen. The CPO reaction being exothermic, heat is produced. At the outlet temperatures of the CPO a corrosion problem called “metal dusting.” may appear. Metal dusting is a catastrophic form of corrosion that occurs when susceptible materials are exposed to environments with high carbon activities. The corrosion manifests itself as a break-up of bulk metal to metal powder. This may shorten the life of the equipment involved, and may lead to contamination problems in end-product obtained from such equipment. Metal dusting for any given material tends to occur in a relatively narrow range (100-300° C.) of a temperature regime from 400 to 900° C. The particular temperature window of susceptibility depends on the material, gas composition, thermodynamic consideration and kinetics. In the case of the typical syngas composition the critical range of temperature is within 450-750° C.
It is believed that the phenomenon of metal dusting in syngas service is caused by the precipitation of carbon into the metals that comprise the material of construction. The mechanism underlying the metal dusting phenomenon is not completely understood, but it is known that the higher concentration of CO in the gas exiting the CPO reactor compared to a steam reforming reactor worsens the metal dusting phenomenon.
The latter phenomenon particularly occurs downstream of the CPO reactor, where the initial syngas mixture is subjected to cooling prior to the water gas shift reaction. As a result of the cooling, the gas passes the temperature range in which metal dusting is bound to occur. The temperature reduction is generally achieved in a Process Gas Boiler (PGB), which is a heat exchanger designed to quickly reduce the temperature of the process gas to a temperature suitable for the water gas shift reaction. Thus, also the PGB should be protected from metal dusting. A known method to reduce this phenomenon is to design the process gas boiler for a minimum residence time, preferably less than 2 seconds, preferably between 0.25 and 1 seconds, more preferably between 0.25 and 0.5 seconds. However, it is practically difficult to design and operate the PGB for lower residence times because lower residence times lead to very high gas velocities which in turn may increase the material problems incurred.
WO2010/144544 A1 discloses a method for producing synthesis gas from biomass. The document discloses solar driven refining of biomass and the lowering of the temperature after the reaction to avoid the reverse reaction by various means, for example by indirect heat exchange or by quenching with water. It is disclosed that rapid cooling e.g. between 0.1 and 10 seconds, may reduce the metal dusting phenomenon. There is no disclosure of the particular problems associated with the use of a catalytic partial oxidation reaction followed by a water gas shift (WGS) reaction, i.e. the increased corrosiveness and the need to adjust the S/G ratio before the WGS reaction. A typical temperature of less than 800° C. is disclosed as required to prevent the reverse reaction and optimize the amount of syngas.
It is thus desired to provide a process which is capable of providing syngas and reduce or avoid conditions that could promote the phenomenon of metal dusting.